Lighter, Clearer, and More Immersive – How the Waveguide – Light Engine Combination Transforms Our View of AR Smart Glasses
March 20, 2024
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When most people hear the term “AR”, they instinctively think of virtual reality (VR) technologies, imagining a bulky and heavy headset that cuts the wearer off from the outside world. VR headsets have tended to be expensive, with a focus on the gaming market, and can make wearers feel self-conscious.
Certainly, there’s an important role for VR to play, but perhaps the more exciting market right now is in the augmented reality (AR) space, where a layer of digital text and images is laid over, or augmented on to, the actual world around us. Thankfully, unlike most VR headsets, modern augmented reality technology can be inconspicuously built into a pair of light, compact and unobtrusive smart glasses, unbeknownst to the world around the wearer.
The key enabling technology of augmented reality is, of course, the projection display itself. This system is a seamless merger of display and light engine technologies. It must be small and ultra-light weight, to ensure that the glasses it is integrated into remain comfortable to wear all day. At the same time, it must produce a clear, bright image that is easy to read both for indoor and outdoor ambient light conditions. And of course, it must consume minimal power for extended battery operating times. Furthermore, the smart glasses display system must be unobtrusive and not distort the view of the real world, being as inconspicuous as possible to both the wearer and other people in the vicinity.
A superior display system needs an optical combiner to route the projected light to the wearer’s eye. There are several optical combiner technologies that may be used with the light engine, of which the most typical see-through display is a waveguide. Its size, features, and technological basis determine whether the dimensions of AR comfort – social, wearable, and visual – are met.
Not all waveguides are created equal, and engineers need a decent understanding of how this component functions and a good grasp of optics in order to make design decisions in this crucial area of augmented reality. In this article, we’ll focus on explaining what a waveguide is, what it does and how it works. We’ll also review the different technical approaches to system-level design of augmented reality smart glasses and investigate how the waveguide forms its integral part.
What is a Waveguide?
The fundamental concept in AR smart glasses is that the user’s view of the real world is augmented with an overlay image, which is projected onto the display. To achieve this effect, we require a so-called “optical combiner”, one of which is a waveguide.
Waveguides are widely used already in mainstream AR devices such as Microsoft HoloLens 2, INMO Air2, and Rokid Vision 2. There are other types of optical combiners, but these are not covered by this article.
A waveguide takes the light created by the light engine, which enters the waveguide via an input grating. The waveguide subsequently bends and directs the light into the human eye (see Figure 1) by utilizing total internal reflection (TIR) to bounce the light inside the waveguide, before it exits via an output grating. This allows the delivery of a composite image that combines the real and virtual visual inputs to the user’s eye.
The process of routing the light is called 2D pupil expansion and enables a small light engine to create a large “eye box”, which means it can be used with different people’s varying head sizes and distances between the eyes and so called the Interpupillary Distance (IPD).
Figure 1: How a waveguide combines the real world with a displayed image. (Source: Seeing is Believing: Exploring the Future of XR Display Technology”, Sam Dale, IDTechEx, https://www.idtechex.com/en/research-report/displays-for-virtual-augmented-and-mixed-reality-2024-2034-forecasts-technologies-markets/955)
The waveguide is usually housed within the frame of the glasses adjacent to the lens. It is typically made from glass and is very thin, meaning that it remains unobtrusive.
What Types of Waveguides Are Used in AR Smart Glasses?
The principle of a waveguide is quite straightforward, but the optics technology behind it is surprisingly sophisticated. While there exist other options, smart glasses predominantly utilize two main types of waveguides: reflective (also known as geometric) and diffractive. These two waveguide types differ in the way that they direct light to the eye:
- Reflective – a combined image is created utilizing a semi-reflective mirror inside the waveguide to reflect light from the image produced by the light engine, whilst simultaneously permitting light to pass from the outside world.
- Diffractive – a diffractive optical element (DOE) made up of slanted gratings, known as surface relief gratings (SRG), on the waveguide’s surface routes the light from the image to the eye and combines it with the incoming light from the real world. First patented by Nokia, this type of waveguide has been widely used in AR glasses[1]. Other types of DOE such as volume holograms are also used as an alternative to SRG.
Figure 2: how a waveguide works.
What Drives the Design Choice for AR?
Before answering this, let’s take one step back and look at the overall technology required to produce an AR image.
The image in AR smart glasses is created by means of a tiny projector, the light engine. This usually either consists of a grid of LED pixels or is created by a laser beam scanner (LBS). The LBS builds the image up by scanning either as a “raster” (similar to cathode ray tube displays) or as a Lissajous pattern, which creates a uniform and high fill-factor projection.
The light engine then needs the optical combiner, in our case a waveguide, to route the light to the wearer’s eye. Waveguides constitute a significant portion of the target weight of consumer AR glasses, which is an important factor in design choice. Additionally, the efficiency of light propagation through the waveguide determines the brightness of the image and influences power consumption. Economies-of-scale and price tag are also key design criteria; hence the mass production-readiness of waveguide technology is an important consideration.
Reflective waveguides are relatively thicker and transparent, typically achieving a high efficiency that leads to a bright image and low power consumption. This type of waveguide is compatible with both monochrome and full color LED projector technologies such as DLP, LCoS, and microLED.
Reflective light waveguide technology is based on the principle of geometric optics, inserting reflector arrays in the waveguide as a technical solution. Its advantage is good imaging quality, but the disadvantage is the complex preparation process of reflector arrays, the difficulty to replicate, the low yield of finished products, and the existence of ghost images.
Diffractive waveguides, on the other hand, offer the desired combination of minimal thickness, very low weight and full transparency to display high-contrast, bright images. These waveguides are compatible with both monochrome and full color LBS projection displays, and allow for both relay optics and direct light in-coupling. Diffractive waveguides support both raster and Lissajous laser beam scanning methods.
The upside in the diffractive lens technology is the ease of mass manufacturing, as it uses laser etching rather than molding, cutting, and gluing of striped glass in the geometric design. The downside with diffractive waveguides is that they tend to reduce the amount of light being sent into the wearer’s eyes. The solution to mitigate this is to have multiple layered diffractive gratings, but this thickens the glass itself, going against the design paradigm of thin, lightweight, and stylish. The good news is that innovations already exist on a single-layer diffractive technology, and diffractive waveguide efficiency is improving with each new generation.
Seamlessly Merging Display and Light Engine Technologies for Lighter, Clearer, and More Immersive AR
Ultimately, the design choice of waveguide is not only driven by the projection technology used in a set of AR smart glasses, but also all dimensions of AR comfort - social, wearable, and visual – as a simple eyeglass-lens form factor.
TriLite’s Trixel® 3 uses an LBS display and is the world’s smallest projector for AR smart glasses – it takes up less than 1cm3 and weighs a tiny 1.5g. Trixel® 3 does not require any external projection optics or relay optics to couple to a waveguide, which saves on weight, size and power consumption.
TriLite combines the Trixel® 3 LBS projector with an ultra-thin diffractive waveguide to produce one of the most compact AR display systems possible – in any size or style. TriLite’s high-brightness LBS supports commercial diffractive waveguides, thereby overcoming a significant barrier to adoption because an accurate, crisp image is achieved even for the brightest outdoor conditions, for example, for alpine skiing glasses.
Watch: TriLite and Dispelix Fireside Chat
A picture-perfect image in AR smart glasses requires precise calibration across the combined display system (projector and waveguide), for which TriLite has developed an effective end-to-end software-based calibration system. This unique software-first approach ensures a fully calibrated image that goes to the eye of the viewer.
Conclusion
Recent developments in diffractive waveguides are showing the way ahead for light, comfortable, and discreet AR smart glasses that provide accurate, crisp, and bright augmented reality images even in the most demanding ambient light conditions.
While there exist multiple possible approaches, we believe that diffractive waveguides are the ideal match for the ultra-compact Trixel® 3 LBS projector, striking the right balance in terms of the size, efficiency, and image quality characteristics that augmented reality system designers are demanding and that end users will appreciate.
Dr. Manuel Dorfmeister is an expert in microelectromechanical systems, with a PhD in Piezoelectric MEMS development from the Technical University of Vienna, as well as numerous patents registered and papers published. In 2021, Manuel was appointed head of the MEMS & Software department at TriLite, the company that is designing and building the world’s smallest projection displays for Augmented Reality mass market devices.
